Alkali metal silicate solutions are mostly composed of water and a distribution of silicate anions of different molecular weights, whose charges are balanced by metal cations and protons. In addition, trace metallic and anionic impurities are present. These impurities originate from raw materials used to manufacture silicate solutions and from erosion of process equipment used in such manufacture. These impurities may be undesirable in applications in which silicate solutions are used. Such applications include the use of silicate solutions in the manufacture of siliceous catalysts, as chromatographic supports, and as cleaning solutions. The impurities may be found on the surface of siliceous supports, thereby undesirably changing the surface properties, such as reactivity and adsorption. In long term storage, the presence of impurities may lead to precipitation of siliceous solids which may remain as undesirable residues in demanding cleaning applications.
Accordingly, it is desirable to purify alkali metal silicate solutions to avoid the detrimental effects of the impurities. A number of methods are known to produce and purify alkali metal silicate solutions of commercial purity, which is a purity level which may cause some or all of these detrimental effects. For example, Table 1 shows some typical commercial alkali metal silicate solutions, with the compositions and impurities shown both on a wet basis (as sold as a concentrate) and on a 100% SiO.sub.2 basis. As used herein, a "commercial alkali metal silicate solution" (or a "feed alkali metal silicate solution") shall mean an alkali silicate solution having an impurity level of at least 450 total ppm of the following impurities Al, Fe, Ca, Mg, and Ti (taken together), measured on a 100% SiO.sub.2 basis. The total of these impurities is listed in Table 1 for each commercial silicate solution as "Sum ppm," and the total of these impurities for the first solution listed (i.e., KASIL.RTM. potassium silicate solution sold by PQ Corporation of Valley Forge, Pa.) is 913 ppm.
One common way of making an alkali metal silicate solution, particularly a sodium silicate solution, is a fusion process. In this process, mixtures of sand and sodium carbonate are fused in open hearth furnaces of regenerative or recuperative design fired with gas or oil. This process would require very high purity quartz sand and sodium carbonate to manufacture alkali metal silicate solutions substantially more pure than those listed in Table 1.
TABLE 1 __________________________________________________________________________ Commercial Silicate Solutions PQ Corp. KASIL .RTM. 6 Zaclon Occidental Potassium Potassium PQ Corp. N .RTM. PQ Corp. RU .RTM. Petroleum 42D Vinings #9 Silicate Silicate Sodium Silicate Sodium Silicate Sodium Silicate Sodium Silicate 2.1 wt. ratio 1.8 wt. ratio 3.22 wt. ratio 2.4 wt. ratio 3.25 wt. ratio 3.11 wt. ratio __________________________________________________________________________ % SiO.sub.2 26 100 26 100 28.7 100 33 100 30.1 100 28.7 100 Al ppm 91 405 198 762 136 473 227 688 190 631 84 212 Fe ppm 39 172 131 504 39 136 58 177 50 166 44 11 Ca ppm 38 168 20 77 11 38 27 82 25 83 18 45 Mg ppm 11 49 8 31 6 20 10 32 12 40 6 15 Ti ppm 27 119 40 154 39 138 52 158 44 146 34 86 Sum ppm 205 913 397 1527 231 805 375 1136 321 1066 186 469 Cu ppb n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 400 1400 Ni ppb n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 100 350 Cr ppb n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 300 1000 Zn ppb n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 1000 3500 __________________________________________________________________________ n.a. means not analyzed
According to U.S. Pat. No. 4,857,290, high purity silica can be produced from a silica sol by first preparing, from an alkali metal silicate solution, an acidic silica sol and using a cationic exchange resin to remove the impurities such as Ti, Fe, and Al. For example, a dilute silicate solution which is contacted with a relatively large amount of resin will remove sodium or potassium to the point where colloidal silica (sol) will form.
The use of ultrafiltration devices, which have membranes with molecular weight cutoffs generally higher than nanofiltration devices, have been mentioned. For example, R. K. Iler has mentioned the use of ultrafiltration to determine molecular weight distribution of a silicate solution, but does not disclose using such membranes to remove impurities from silicate solutions. In addition, U.S. Pat. No. 4,857,290 mentions the use of ultrafilitration as a procedure to purify an alkali silicate solution prior to the preparation of the acidic silica sol, but does not envision withdrawing the permeate of this procedure as product alkali metal silicate solution, without further processing.